The present invention relates to apparatus for controlling the load angle of a converter with a three-phase output.
A preferred field of application is the operation of a frequency converter-fed rotating-field machine with phase angle control for the stator currents. The frequency converters used advantageously have a d-c input, the d-c current of which is regulated or controlled to a reference value for the amplitude of the frequency converter output currents. Such an arrangement is shown in FIG. 1, as is described in DE-OS 29 19 852.
With respect to FIG. 1, the rotating-field machine M (an asynchronous machine in this case) is connected to an inverter WR on the machine side, the d-c inputs of which are connected to a rectifier GR on the network side via an intermediate d-c link. This rectifier GR on the network side is controlled by a control unit STR, the control voltage of which is connected to a current controller RI for the amplitude I of the output currents or the intermediate link d-c current. The corresponding reference current value I* can be taken off via a function generator FG at the output of a speed controller RN. In DE-OS 29 19 852, it is described that the output signal of the speed controller RN is proportional to the tangent of the load angle, i.e., proportional to the quotient of the components of the stator current perpendicular and parallel to the flux. This is therefore a signal similar to an angle which, for a given flux (i.e., for a predetermined reference value for the stator current component parallel to the field) is the reference value wI(F)* for the angle between the stator current vector and the flux vector of the machine. An actual value computer CAL forms an actual value wI(F) which is fed to the actual-value input 1 (negative input of summing stage) of an angle controller RW. The reference value input 2 (positive input of summing stage) of the angle controller is connected to the output of the speed controller RN (in the general case, an input device for an angle-like reference value). The output of the angle controller RW is connected to the frequency control input 3 of the control unit STW which controls the frequency converter WR on the machine side.
The known actual-value controller is connected via 3/2 converters to measuring devices MI and MU which measure the voltage and the current of the machine and form therefrom the components of the corresponding voltage vectors and current vectors in a reference system referred to the stator. The actual-value computer CAL comprises a flux computer CAL F which calculates the direction of the flux axis, i.e., practically the angle wF(S) between the flux axis and the stator axis, and on the other hand the flux-parallel component of the stator current vector. The components of the stator current vector parallel and perpendicular to the flux axis to be calculated can then be formed from the output signals of the current measuring device MI and the quotient of the two stator current components practically determines the angle wI(F). This angle wI(F) is therefore equal to the angular difference between the angle wI(S), which the stator current vector impressed by the frequency converter WR encloses with the stator axis, and the calculated angle wF(S) between the calculated field axis and the stator axis. It represents the actual value of the angle controller RW.
Considering only the angles, the known actual-value computer operates according to the principle shown in FIG. 1:
A measuring member MW for the angle wI(S), i.e., an angle detector, forms the actual angle value wI(S) for the phase of the frequency converter output currents which is referred to the stator axis. Since the reference angle value wI(F)* of the angle controller RW is referred to the flux axis, i.e., the reference angle wF(S), the actual value wI(S) and the reference value wI(F)* must be referred by a suitable phase shifting member to a common reference value (in this case, the phase of the flux). A subtraction member could therefore serve as the phase shifting member PH, which forms the difference wI(S) - wF(S) between the actual angle value wl(S) and the reference value wF(S).
To the speed controller RN must be connected, besides the reference speed value, a suitably determined actual speed value. In DE-OS 29 19 852, the frequency control variable fI(S)* is fed back as a substitute actual value for the speed from the frequency control input 3 of the control unit STW for the frequency converter WR on the machine side. By the switching processes in the frequency converter WR, however, pulse-like currents are fed to the stator winding in order to distribute at high frequency, the d-c current I impressed by the rectifier GR at equidistant points in time to different combinations of the three-phase inputs of the machine. At low frequencies, the individual phase currents are pulsed with a different duty cycle. In the block diagram of the control unit STW, this is indicated by the provision that the frequency control signal fI(S)* is integrated in an integrator INT to form a corresponding control signal WI(S)* for the stator referred phase angle which is impressed on the stator currents via the frequency converter WR by means of pulse width modulation (modulator MOD). The pulse-like stator currents, however, bring about that the actual angle value wI(S) is rich in harmonics, especially in pulsed operation at low frequencies. This is a disadvantage particularly if digital control without determination of the speed is used. Also in other cases it may become necessary to filter out the high frequencies in the actual angle value wI(S), where the required smoothing must be set very high in many cases in order to arrive at stable operation. This, however, degrades the desired large dynamic range of the angle control circuit considerably.